Because of their tremendous capacity for in vitro expansion and ability to differentiate into phenotypically unambiguous cardiomyocytes, pluripotent human embryonic stem cells (hESCs) are an attractive source for cell-based cardiac therapies. Our group has exciting new data indicating that hESC-derived cardiomyocytes (hESC-CMs) can couple with host myocardium following transplantation in guinea pig hearts, but their integration is imperfect in injured hearts. We have also found that hESC-CM transplantation significantly decreases the incidence of both spontaneous and induced arrhythmias. The present application builds on these observations and has two overall goals: first, to determine the mechanistic basis for this arrhythmia-suppressive effect and, second, to test novel approaches to further enhance the electromechanical integration of hESC-CMs in injured hearts. In Aim 1, we will test the hypothesis that the beneficial effects of hESC-CM transplantation on electrical stability correlates with their functional incorporation. In Aim 2, w will test the hypothesis that treatment with gap-junction modifiers will improve host-graft coupling following hESC-CM transplantation in a guinea pig infarct model. Finally, in Aim 3, we will use in vitro models to develop Wnt5a-mediated chemotaxis as a complementary strategy to improve the integration of hESC-CM grafts.